Apparatus for processing particulate material



Dec. 13, 1966 T. A. J. JAQUES ETAL 3,299,723

APPARATUS FOR PROCESSING PARTICULATE MATERIAL Filed Oct. 25, 1963 5 Sheets-Sheet l Dec. 13, 1966 T. A. J. JAQUES ETAL 3,290,723

APPARATUS FOR PROCESSING PARTICULATE MATERIAL 5 Sheets-Sheet :5

Filed Oct. 25, 1963 1966 T. A. J; JAQUES ETAL 3,290,723

APPARATUS FOR PROCESSING PARTICULATE MATERIAL Filed 001;. 23, 1965 5 Sheets-Sheet 2 41 K3530 8942 4?) 9 hm Hm United States Patent D 3,290,723 APPARATUS FOR PROCESSING PARTICULATE MATERIAL Thomas Arthur John Jaques, Radipole, Weymouth, Dorset, Derek William James Sturge, Dorchester, Dorset, and Richard Terrance Smyth, Winton, Bournemouth, England, assignors to United Kingdom Atomic Energy Authority, London, England Filed Oct. 23, 1963, SenNo. 318,246 Claims priority, application Great Britain, Oct. 26, 1962, 40,569/62 8 Claims. (Cl. 18-4) This invention relates to methods and appartus for making spherical particles of refractory materials and use is made for this purpose of a plasma generator.

Plasma generators are Well known in technology associated with high temperature laboratory techniques for producing very high temperatures which cannot easily be achieved by other means. Moreover their use for utilitarian processes where a high temperature environment is necessary has also been suggested in connection with several techniques involving melting of materials. One such technique previously proposed related to the manufacture of spheroidal powder and this process comprised the step causing the efliuent of a gas plasma from an electric arc to impinge upon a rod of material, of which the powder is to be formed, so causing the rod to be spattered into spherical particles by the combined effects of the kinetic energy and the heat from the gas stream.

Such a process is quite uncontrolled and produces particles having a wide range of diameters together with random dispersion of sizes within the range. Moreover physical distribution of the particles over a wide area means that the system is unsuitable for a controlled production process such as the manufacture of refractory powder within a narow size range, for example the manufacture of nuclear fuel refractory particles which are presently considered to be preferred at least 150 microns diameter.

Processes proposed hitherto for this purpose do not appear to be entirely satisfactory when applied to forming spheres whose diameter is 150 microns, or above, on a production scale. It is an object of this invention to provide an improved process for making spheres out of particles of refractory materials.

According to the invention a process for spheroidizing particulate material having a high melting point comprises introducing the particles into a plasma-forming Work gas and passing the gas together with the entrained particles through a zone in which the gas is ionised to form 'a hot gas plasma whose temperature is such as to melt the particles which then assume a substantially spherical shape, allowing the particles to pass from said Zone and then cooling the particles whilst the assumed shape is preserved.

Preferably the particles are cooled at least partially in free flight in a space into which the melted particles pass from the zone.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

FIGURES 1 and 2 show alternative forms of spheroidising apparatus;

FIGURE 3 shows one form of plant for the production of spherical particles employing one form of the plasma generators shown.

Both the views FIG. 1 and 2 are shown as axial crosssections and both apparatuses are generally symmetrical about their axes X-X.

In FIG. 1, a container 1, within which a pressure lower than atmospheric can be produced, has a feed tube 2 for connection to a supply 2a of particulate material of irice regular surface contour at the top and an outlet aperture 3 at the bottom. Adjacent the inlet tube 2 is supported a plasma generator 4 which supplies sufficient heat to the particles to melt them.

The plasma generator 4 comprises an anode 5 mounted at one end of a short tube 7 of insulating material which has an internal shoulder 7a. p The anode 5 is in the form of a grooved copper ring 8 which seats on the upper face of the shoulder 7a with the open rim of the groove flush with the end face 7b of the tube 7. An end plate 9 attached to the face 7b by studs 10 closes the groove in the ring 8 and so forms a cooling channel 11 for the anode. The plate 9 is pierced by coolant inlet and outlet 12, 13 and also by a central aperture 14- sleeved with an insulator 15 for receiving the feed assembly. The feed assembly 6 comprises an inner tube 6a connected to a supply of particles via tube 2 and an outer concentric and co-axial tube 6b connected to a supply 6c of work gas. Sealing rings provide a gas tight seal between abutting faces of the plate 9, tube 7 and grooved ring 8. Fixed to the inner diameter of the ring 8 is a spacer 8a of insulation material such as Tufnol (RTM) from which depends a ring 16 of graphite. The outlets of the tubes 6a, 6b are thus shrouded by the ring 16.

Into the end of the tube 7 remote from the plate 9 is fitted an annular member 17 which has spaced flanges 17a, 17b and a central tubular portion 17c of reduced diameter forming a constriction.

The flanges 17a, 17b are of such a diameter that the flange 17a, which has a peripheral sealing ring 18, can enter the tube 7 and abut the shoulder 7a whilst the flange 17b abuts the end face of the tube 7 to which it is attached by screws studs 19. A sealing ring 20 enables a satisfactory seal to be made for an annular chamber 21 formed between the periphery of the tubular portion 17c, the flanges 17a, 17b and the inner face of the tube 7.

The internal surface of the tubular portion is lined with a ring 25 graphite or other suitable refractory to define an outlet nozzle and to present a construction to the gas flowpath extending from the electrode 16'to outlet port 3. The wall of the tube '7 is pierced by apertures (not shown) which communicate the chamber 21 with coolant flow and return pipes 22, 23, and also by a penetration 27 for electrical supply cable 2711 connected to anode 8.

The anode of the plasma generator may be formed by the graphite or a suitable refractory metal ring 25. Alternatively the nozzle can act as a collimator and the cathode is then provided by a water cooled tungsten member 28 spaced axially from the nozzle and offset from the center line thereof.

Alternatively the electrode ring 16 can be made the cathode and the nozzle 25 the anode whilst a tungsten rod 28 is connected as a stabilizing cathode at the same potential as ring 16. This has the effect of influencing the plasma flame F towards that side of the container 1.

In operation the container 1 is reduced in pressure down to 10 mm. Hg a supply of work gas is applied to the plasma generator and a sufficient voltage, in relation to the reduced pressure in the chamber and the distance apart of the electrodes, applied across the cathode and anode to sustain a gas plasma.

Particles of refractory materials such as carbides of nuclear fuel metals are prepared by any known pelleting process so as to have a volume corresponding to that of a spherical particle havingthe required diameter and delivered to the supply 2a.

The particles to be spheroidized are fed from the supply 2a through the feed tube 6a. The particles emerge from the tube 60 and become entrained in helium gas supplied through pipe 6b. Thus entrained the suspension passes through the annular electrode 16 and enters an axially extending flowpath. The electrodes having been energized the gas becomes ionized as the suspension passes through the constriction presented by the nozzle so that the particles melt in the resultant plasma. The supplies to the electrodes varies with the distance they are apart and also the gas being used e.g. argon at 80 V. DC. Open circuit would be increased to 160 V. DC. with helium. Once melted the particles assume spherical shape and pass into a particle collecting zone.

An alternative form of apparatus shown in FIG. 2 has a different form of plasma generator comprising a composite body one part of which provides a flange 31 by which the body 30 is secured to a tubular container 32.

The other parts of the body 30 include a ring 33 and an annular plug 34, the latter being of electrically insulating material. The plug 34 is secured to the flange member 31 by screws 35 and to the ring 33 by screws 36. The ring 33 has dependent tubular portions 33a, 33b. The plug 34 enters the space between the portions 33a, 33b and defines an annular space 33c. Coolant liquid is circulated through space 330 between the two tubular portions 33a, 33b, the liquid being admitted through pipe 37 and port 33a in the portion 33b and withdrawn via port 38b. The bore of the ring 33 is axially aligned with a bore 41 in the plug and part of the bore in the ring is screw threaded to receive a tube 39 having a bore 39a. The latter provides a feed passageway for particles of high melting point material and is staggered off the axis of the body near its lower end at 3%. The lowermost part of the tube 39 can support an anode 40 depending within an enlarged diameter portion 41a of bore 41 in the plug 34.

The cathode with respect to the anode 40 is a flanged tube 42 a having a small diameter flange 42a which enters the enlarged diameter bore portion 41a and a larger diameter flange 42b which abuts and is secured to, the lower face of the flange member 31.

The part of the cathode 42 between the flanges 42a, 42b is of such a diameter as to extend through the flanged member 31 with clearance so as to define an annular space 43. The flanged member is pierced by a conduit 44a through which coolant fluid is supplied to the space 43 to cool the cathode structure, the coolant leaving the space 43 via conduit 44b.

The enlarged diameter bore 41a of the plug is lined with a ceramic insert 45 which serves to protect the internal parts of the body against the effects of high temperature. The plug is pierced by a duct 46 and the insert tube 45 is similarly pierced to allow work gas to be admitted to the enlarged diameter portion 41a via tube 46.

In operation of the apparatus shown in FIG. 2, the appropriate electrical connections 47, 48 are made to the anode and cathode respectively and the container 32 is evacuated down to 3 p.s.i Hg. A work gas having a good heat transfer property, such as helium, is passed through tube 46 to enter the enlarged diameter portion of the plug with a swirling motion about the axis of the anode 40.

Particulate material to be spheroidised is poured into the upper end of the bore 39a of the tube 39 and this passes down the tube 39 and into the swirling gas stream. Thus entrained, the particulate material is drawn through a plasma arc struck between anode and cathode and projected through the bore of the tubular member 42 into the container 32.

A completed apparatus suited to small production runs is illustrated diametrically in FIG. 3.

In FIG. 3 the plasma generator 50 which may be either of the types described in FIG. 1 and 2, is mounted at the upper end of a cooling column 51 which is connected to receive the particles made molten in the plasma flame. The wall of the column is cooled by a helical tube 52 attached to the outer surface of the wall and through which water can be passed, being admitted via pipe 53 and passed to drain via pipe 54. The lower part of the wall is funnel shaped and directs the particles, cooled free flight, into a tube 55 depending into a portable vessel 56 containing cooling oil. The interior of the column is connected via duct 57 to a vacuum pump 58 in order to reduce the pressure within the apparatus to 10 mm. Hg.

The view in FIG. 3 shows the plasma generator 50 externally. It has connections 59, 60, 61 and 62 for inlet and outflow of coolant liquid and an inlet pipe 63 for work gas which is supplied from a cylinder 64.

It is preferred to use helium for a work gas on account of its high thermal conductivity but any other suitable gas may be employed instead.

As an example of the product made in the apparatus described, particles of uranium and thorium carbides were processed using helium gas at 28 litres/ minute.

We claim:

1. A process for spheroidising particulate material which resides in preparing a batch of irregularly shaped particles the volume of each approximating to that of a spherical particle of the required diameter, causing the particles to become entrained in a stream of inert gas having a high heat transfer coefficient, flowing the gas with its particles entrained therein along a flowpath, initiating a zone of hot gas plasma through a constriction in the flow path whilst an electrical potential is maintained across the constriction in said flowpath to melt the particles whereby the latter assume a spherical shape, passing the melted particles in free flight to a cool zone and collecting the particles in the cool zone and including the step of initially reducing the pressure in the flow path below atmospheric before introducing the gas with entrained particles.

2. A process as claimed in claim 1 in which the electrical potential sufficient to ionize the gas is maintained between an annular electrode forming the constriction and a central electrode projecting into the annular electrode.

3. Apparatus for spheroidizing particulate material of a refractory nature comprising means for supplying an inert gas having good heat transfer properties, means for supplying particles of irregular shape downwardly along a vertically dependent tube, each particle having a volume approximate to that of a spherical particle of the required diameter, a vertically extending chamber, conduit means for communicating both said means with the chamber to induce therein a suspension of the particles in the inert gas, a vertically extending flowpath communicating the chamber with a coded vertically extending particle collecting region, means for reducing the pressure in the flowpath below atmospheric before introducing the gas to entrained particles a constriction within the vertical flowpath, electrodes connectable to electrical supplies for ionizing the gas in said constriction.

4. Apparatus as claimed in claim 3 in which the conduit means includes a conduit including an inlet port through which particles are introduced into the chamber, along with the gas, said inlet port, the constriction in said flowpath and said electrodes being arranged asymmetrically with respect to the axially extending flowpath.

5. Apparatus for spheroidising particulate material of a refractory nature comprising means for supplying particles of irregular shape, means for supplying inert gas, means for feeding the particles under gravity along a vertically dependent tube, an annular electrode supported around the outlet of said tube arranged to direct the particles into a chamber, an annular conduit disposed around the tube for leading gas from the supply to said electrode, a flowpath leading from the annular electrode to a cool particle-collecting region, a constriction in said flowpath downstream of the first annular electrode, formed by a second annular electrode, and a third, stabilizing electrode mounted in the flowpath on the downstream side of the constriction.

6. Apparatus as claimed in claim 5 in which the first and second annular electrodes are co-axial whilst the third electrode is eccentric to the axis.

7. Apparatus as claimed in claim 6 comprising coaxially extending inlet pipes for particles and inert gas, the pipes terminating within an annular cathode, an annular anode co-axial with the cathode and spaced axially therefrom and a stabilizing cathode spaced axially from the anode on the downstream side thereof eccentric to the common axis to stabilize wandering movement of the plasma flame on to a point ofiFset from the axis.

8. Apparatus for spheroidising particulate material of a refractory nature comprising a supply of particles of irregular shape, a supply of inert gas, means for feeding the particles under gravity along an axially extending bore of a vertically dependent tube, an elongated electrode, means for supporting the electrode so that it depends from the pipe on the axis of the bore, structure defining a chamber having a cylindrical wall co-axial with said electrode, a particle inlet passage communicating the tube with the chamber asymmetric with respect to the axis of the chamber wall, a gas inlet passage in the chamber wall, conduit means for directing the gas from said supply to the interior of the chamber, through said gas inlet, a vertically extending outlet passageway extending through the chamber Wall, an annular electrode in said passageway defining a constriction to flow therethrough, means for energising the electrode to ionize the gas in said constriction, whereby the particles are heated to fusion temperature and form spheroids, and a cool particle-collecting region below the constriction.

References Cited by the Examiner UNITED STATES PATENTS 2,676,892 4/ 1954 McLaughlin.

2,736,713 2/1956 Murray et a1.

2,911,669 11/1959 Beckwith 26415 3,010,009 11/1961 Ducati 21976 3,015,852 9/1962 Hoifman et al 26415 3,075,066 1/1963 Yenni et a1 21976 3,151,965 10/1964 Patterson 26415 X 3,171,714 3/1965 Jones et al 26415 X 3,183,377 5/1965 Winzeler et al 219-76 3,197,810 8/ 1965 Bildstern.

WILLIAM I STEPHENSON, Primary Examiner. 

1. A PROCESS FOR SPHEROIDISING PARTICULATE MATERIAL WHICH RESIDES IN PREPARING A BATCH OF IRREGULARLY SHAPED PARTICLES THE VOLUME OF EACH APPROXIMATING TO THAT OF A SPHERICAL PARTICLE OF THE REQUIRED DIAMETER, CAUSING THE PARTICLES TO BECOME ENTRAINED IN A STREAM OF INERT GAS HAVING A HIGH HEAT TRANSFER COEFFICIENT, FLOWING THE GAS WITH ITS PARTICLES ENTRAINED THEREIN ALONG A FLOWPATH, INITIATING A ZONE OF HOT GAS PLASMA THROUGH A CONSTRICTION IN THE FLOW PATH WHILST AN ELECTRICAL POTENTIAL IS MAINTAINED ACROSS THE CONSTRICTION IN SAID FLOWPATH TO MELT THE PARTICLES WHEREBY THE LATTER ASSUME A SPHERICAL SHAPE, PASSING THE MELTED PARTICLES IN FREE FLIGHT TO A COOL ZONE AND COLLECTING THE PARTICLES IN THE COOL ZONE AND INCLUDING THE STEP OF INITIALLY REDUCING THE PRESSURE IN THE FLOW PATH BELOW ATMOSHERIC BEFORE INTRODUCING THE GAS WITH ENTRAINED PARTICLES. 